Question

As per critical relative humidity (CRH), the most hygroscopic fertilizer is

• A. Ammonium sulphate
• B. Urea ammonium sulphate
• C. Ammonium nitrate
• D. Ammonium chloride

Hint:

Lower critical relative humidity means a fertilizer absorbs moisture more easily.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Critical relative humidity (CRH) is the humidity level at which a fertilizer starts pulling moisture from the air. A lower CRH means the fertilizer starts absorbing moisture at a lower humidity, which makes it more hygroscopic, that is, more prone to caking and going sticky in storage.

Step 2: Key Formula or Approach:
Compare the typical CRH values of the four fertilizers listed, at a fixed temperature such as 30 degrees Celsius, and pick the one with the lowest value, since the lowest CRH marks the most hygroscopic material.

Step 3: Detailed Explanation:
Ammonium sulphate has a CRH of around 79 to 81 percent, fairly high, so it stays free flowing under most ordinary storage humidity.
Ammonium chloride sits in a similar high range, close to 79 percent, so it is also fairly resistant to moisture pickup.
Urea ammonium sulphate is a blended product, and while blending can lower the CRH of a mixture compared with either pure component, its value still does not drop as low as that of ammonium nitrate.
Ammonium nitrate has a notably low CRH, around 59 to 65 percent depending on temperature, well below the other three. This is exactly why ammonium nitrate is difficult to store, needs special coating or blending, and has largely been replaced in Indian fertilizer use by calcium ammonium nitrate, which has a safer, higher CRH.

Step 4: Final Answer:
Since ammonium nitrate has the lowest CRH among these four, it is the most hygroscopic, so option 3 is correct.